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Old 09-03-2017, 11:20 PM   #1 (permalink)
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Raising the rear?

I don't have a real world application for this so I guess it goes in the hypothetical column. If you had a car with a sloping rear but it sloped beyond the "template" could a gain be made by raising the rear (and possibly lowering the front) to better conform to the template? It would increase the height and I assume punch a bigger hole in the air but would there be any gain from better airflow over the top? Just a thought. Probably a simple answer but I like to try and learn when I can.

These are about the only real world examples I can think of.

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Old 09-03-2017, 11:36 PM   #2 (permalink)
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If you raise the back, lower the front or both you create more down-force. The smaller gap from the lower front makes the air expand under the back and "sucks" down the back of the car. I think that alone would counteract any gains seen from making it adhere better to the template.

I think there is still merit to the thought though. If you made a belly pan parallel to the ground at the same height as the front bumper it would eliminate the vacuum effect at the back. It would then conform to the template better. But it might increase the Frontal area a little bit. Too many variables to say yes or no I would think.
I know there are others on here more knowledgeable than I that could verify or deny my initial thoughts.

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Old 09-04-2017, 04:09 AM   #3 (permalink)
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Hmm....I'd pondered if raising the rear would be benefitial to give you that 4-degree rise to the underbody. That probably wont work without a completely flat underbody, mind you.
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Old 09-09-2017, 01:35 PM   #4 (permalink)
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inclination

The 1983 Ford Probe-IV concept (Cd 0.152) had active suspension to drop the nose and lift the tail on the highway,for minimum drag.They had the benefit of a wind tunnel to dial in the optimum inclination.
Other vehicles with active suspension which come to mind are:
McLaren P1
Tesla Model S P85
Jaguar XJ220
Bugatti Veyron 16.4
1985 Mitsubishi Galant
Subaru XT (static testing(
Renault Vesta-II
1984 Lincoln Town Car
1970 Porsche 914 (static testing)
1958 Buick Limited Series 700
2017 Ford GT
2008 Ford Fusion 999 LSR (static drop)
GM Ultralite
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Old 09-09-2017, 10:36 PM   #5 (permalink)
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I had a van with torsion bar front suspension and leafs in the rear. I cranked down the front suspension to give me a bit of rake.

The lowered front end acted like an airdam reducing under car drag.

Hucho shows that filling the wake does reduce drag, so the idea has merit.
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Old 09-11-2017, 12:18 PM   #6 (permalink)
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If you do a lot of aerodynamic testing like Porsche did in the early 1970's you might be able to come up with a large Gurney Flap at the rear of the car that lowers Cd and raises top speed by also increasing down-force (Ducktail).

My guess is that just guessing at it will have unpredictable results.

However, the general idea I see is that anything you do to fill out the template, the better off you are.

Link to related thread - see numbers on chart I posted.
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Old 09-12-2017, 01:23 AM   #7 (permalink)
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Any good book on aerodynamics will discuss angle of attack. There is an optimum angle and height above the ground for a body.
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Old 09-12-2017, 02:20 AM   #8 (permalink)
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My grand mother had a '61 GMC 1/2 tone pickup with torsion bar front suspension that was miss alined from the factory. Too low (had a hevy rake) and set for handling/low resistance. After the first alignment to factory speck ,she ran 1 tank with a loss of MPG (I don't remember how much1-3MPG) and the truck handled like a raidal-flyer. She took it back had them reset the truck to the previous wrong seting and recorded the proper specks. Regained the lost MPG and. handling . The truck was maintained like that till '06 when she passed.
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Old 09-12-2017, 02:58 AM   #9 (permalink)
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Don't forget, you're also moving the highest point of the vehicle backwards. This would affect the boundary layer thickness on the roof as well as reducing the rear glass angle, perhaps for more improvement than you would otherwise think
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Old 09-12-2017, 09:36 AM   #10 (permalink)
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I stumbled over this thread recently.

Stepped shape for flow attachment.

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